Double-peaked impact force of very soft gel balls.

We measure the time change of the contact force F(t) during the impact between very soft gel balls and a rigid substrate. For low-impact velocities Vi, F(t) is single-peak functions as intuitively expected; and the relation between Vi and the peak value Fm [of F(t)] obeys the prediction of the standard theory for the impact in the linear elastic regime (i.e., the Hertz theory). On the other hand, for large Vi, where the gel ball deforms into thin pancakelike shapes, F(t) becomes double-peak functions. We compare the data of F(t) for large Vi with a prediction of a model proposed in our previous study [Tanaka, Europhys. J. E 18, 95 (2005)]. The model can quantitatively reproduce the experimental Fm - Vi relation, and shows that the double-peak behavior of F(t) is a consequence of the expanding deformation of the pancake-shaped gel, i.e., spreading motion parallel to the substrate.